Every old bench has a drawer of connectors that "worked fine on the ground." A cold solder joint is sneaky that way. It passes current on the bench, then heats up under full throttle, the resistance climbs, and one day your plane goes quiet halfway through a loop. I've pulled apart enough crashed airframes to know the cause is often a dull, lumpy blob where the battery lead meets the connector.
Good joints aren't hard. You need enough heat, clean tinned parts, and a steady hand while the solder sets. This is how we solder EC3, EC5, IC3/IC5 and XT60 connectors at the builders' bench, with the tools that make it easy and the mistakes that make it hard.
The Short Answer
To solder RC power connectors without cold joints, use a temperature-controlled iron of about 60 W with a chisel tip, tin the wire and the solder cup separately, then join them and hold everything still until the solder sets.
- Heat is the cure for cold joints. A small iron can't heat a 10 AWG wire and an EC5 pin fast enough.
- Lock the pins in place. Plug the connector into its mate (or a soldering jig) while you work, so softened plastic can't let the pins drift.
- IC3 and IC5 plug into EC3 and EC5. Soldering is the same. Just keep the Smart data pin clear.
- One lead at a time, heat shrink on first, and check polarity before the first plug-in.
1. Know Your Connector, Then Pick the Right Iron
Big connectors are mostly a heat problem. The pin and wire act like a heatsink, pulling heat away from the tip as fast as the iron can supply it.
EC3, EC5, IC3, IC5 and XT60 at a glance
| Connector | Typical use | Typical wire | Notes |
|---|---|---|---|
| EC3 / IC3 | Park flyers, 3S planes, 1/10 cars | 13 AWG (E-flite EC3 leads) | IC3 adds a data pin for Spektrum Smart batteries and plugs into EC3. |
| EC5 / IC5 | Larger planes, 4S to 6S packs, big trucks | 10 AWG (E-flite EC5 leads) | IC5 plugs into EC5. Big contacts need a lot of heat. |
| XT60 | Multirotors, chargers, many planes | 12 AWG | Holybro lists 30 A continuous and 60 A burst. Pins are molded into the housing, so watch the heat. |
IC vs EC: same barrel, extra brains
Spektrum's IC3 and IC5 connectors are built to work with the EC family. Horizon's product notes say IC3 and IC5 are compatible with EC3 and EC5 and that the series is fully backward compatible, so Smart batteries work with older systems. The IC version adds a data contact for the battery's chip, so Smart chargers can read the pack. When you solder an IC5 connector, keep solder and heat away from that data contact.
The iron: watts and temperature control
The Adafruit Guide to Excellent Soldering calls 25 W the minimum for small electronics and recommends 50 W or more for a professional, temperature-controlled station. E-flite's EC connector instructions call for a 40 W iron. For EC5 and 10 AWG wire we use 60 W. Both the Trinity TK950 Digital Soldering Station and Trinity TK955 Digital Soldering Station are 60 W, adjust from 200 to 480°C (392 to 896°F), and come with chisel and pencil tips. Use the chisel: more contact area means more heat into the joint.
Solder and flux
Adafruit calls 60/40 rosin core the easiest solder to work with. Lead-free solder works too but needs more heat. Keep a tin of Rosin Paste Flux, 2 oz for big wire, because oxidized strands take solder much better with fresh flux. Work in a ventilated space and wash your hands afterward.
2. The Procedure: Tin, Fill, Join, Hold Still
Every good power joint follows the same steps. Here's the order I teach at the bench.
Step by step
- Disconnect everything. Unplug the battery and work on one lead at a time so the cut ends can never touch.
- Slide on heat shrink first. Cut a piece long enough to cover the solder cup and a few millimeters of insulation, and push it well up the wire, away from the heat.
- Strip only what fits the cup. The strand should sit fully in the cup with no bare copper showing. E-flite's EC instructions call for about 2 to 3 mm on their connector pins, so match the strip length to your cup depth.
- Tin the wire. Twist the strands, add a touch of flux, then heat the wire and feed solder until it wicks all the way through. The wire should look shiny and stay flexible above the joint.
- Lightly tin the cup. Fill it partway, not to the brim. Too much and the joint spills down the outside of the pin.
- Join them. Heat the cup, push the tinned wire in as the solder melts, and hold the iron until the solder flows around the strands. Then remove the iron and hold still until the solder sets.
- Inspect, then shrink. You want a smooth, shiny fillet from wire to cup. Slide the heat shrink down and shrink it with a heat gun, not the iron.
EC3, EC5, IC3, IC5: pins go in after soldering
The EC and IC families use loose gold pins that click into the plastic housing after soldering. Per E-flite's instructions, female tubes go on the battery side and male pins on the device or charger side. While the pin is still warm, push it into the back of the housing with a 1.5 mm hex driver or small screwdriver and firm pressure until you feel it pop into place. If it isn't fully seated, it will push out the first time you plug in.
XT60: let the plastic stay cool
XT60 cups are molded into the housing, so heat goes straight into the plastic. Plug the connector into a mating XT60 before soldering. The mate holds the pins straight if the nylon softens. Work fast with a hot iron instead of slowly with a cool one, and let each side cool before you do the next. Our XT60 male and XT60 female packs come four to a bag for practice.
Use a jig
The Spektrum Soldering Jig holds IC2, IC3 and IC5 connectors and 2 to 7 mm bullets in a mating socket, with a clamp arm for the wire. That's the same trick as using a mating plug, but with your hands free. If you solder more than a couple of connectors a year, it pays for itself.
3. Spotting (and Fixing) Cold Joints Before They Fly
Inspect every joint before you trust it. A little extra time on the bench is cheaper than a crash.
What a bad joint looks like
The Adafruit troubleshooting guide describes a cold joint as one where the solder never fully melted, leaving a rough or lumpy surface. A disturbed joint moved while it was setting and looks frosted or crystalline. An overheated joint has burnt flux on it. A good joint is smooth and shiny, with solder flowing into the strands.
The fix
Cold or disturbed: add a touch of flux and reheat until the solder flows, then hold still while it cools. If the wire was never tinned properly, desolder, cut back to fresh copper and start over. Overheated: clean off burnt flux with isopropyl alcohol and check that the housing isn't warped. Throw out any warped connector. A loose fit has more resistance, and more resistance means more heat.
Polarity and the first plug-in
Before you connect a battery, check that red goes to positive and black to negative on both ends, and use a meter if you have one. Plug in with the plane restrained and the prop off. Then check the joints after the first few flights. A joint that's warm to the touch after landing needs to be redone.
Should you switch connectors?
Standardize on one connector per battery size so packs work across your fleet. If you're converting, adapters such as the EC5 battery connectors let you change the battery side once and be done. A pile of adapter cables just adds more joints that can fail. Our battery charging guide covers charge-lead safety.
Conclusion
Cold joints come from too little heat, dirty parts or movement while the solder cools. Use a 60 W temperature-controlled iron with a chisel tip, tin both parts, hold everything still, and lock the pins in a mating plug or jig so the housing can't warp. Inspect every joint before it flies. Your connectors will outlast your airframes, which is how it should be.
Shop soldering supplies and connectors, or bring your batteries and ESC to the store and we'll walk you through your first joint at the bench.
Gear Mentioned in This Guide
Trinity TK955 Digital Soldering Station
60 W, 200 to 480°C, push-button digital temperature. Chisel and pencil tips included.
Shop now
Trinity TK950 Digital Soldering Station
60 W with a simple dial. Plenty of heat for EC5 and XT60.
Shop now
Spektrum Soldering Jig
Holds IC2/IC3/IC5 and 2 to 7 mm bullets so pins can't drift while the plastic is hot.
Shop now
Spektrum IC5 Device and Battery Connector (1 of each)
One device and one battery side IC5 connector.
Shop nowFrequently Asked Questions
What soldering iron do I need for RC connectors?
A temperature-controlled iron of about 60 W with a chisel tip. E-flite asks for at least 40 W for EC connectors, but EC5 and XT60 on 10 to 12 AWG wire go much more smoothly with 60 W. The Trinity TK950 and TK955 are 60 W stations.
How do I tin wires?
Strip the wire, twist the strands, add a little flux, then heat the wire with the iron and feed solder into the strands (not onto the tip) until it wicks through and looks shiny. Tin the solder cup separately, then join the two.
Are IC5 and EC5 connectors compatible?
Yes. Horizon says IC3 and IC5 connectors are compatible with EC3 and EC5. The IC versions add a data contact for Spektrum Smart batteries.
How do I keep XT60 pins from moving while soldering?
Plug the connector into a mating XT60 or a soldering jig before you heat it. Use a hot iron for a short time instead of a cooler iron for a long time, and let each pin cool before doing the next.
What does a cold solder joint look like?
Rough, lumpy or dull, often with solder sitting on the wire instead of flowing into it. A disturbed joint looks frosted or crystalline. Reheat with flux until the solder flows smoothly, then hold still while it cools.
What wire gauge goes with EC3, EC5 and XT60?
As a guide, E-flite supplies EC3 leads in 13 AWG and EC5 leads in 10 AWG, and Holybro pairs XT60 with 12 AWG. Match your ESC and battery leads, and never go thinner than the leads already on the battery.
